The hepatotoxicity of acetaminophen and a few other known hepatotoxins is mediated not by the parent compound but by a reactive metabolite generated through the cytochrome P-450 enzyme system. The extent of hepatic necrosis is positively correlated with the degree of tissue glutathione depletion. The reactive metabolite has great affinity for reduced glutathione and once this is depleted, it covalently binds to tissue macromolecules causing cell death. We have observed that the hepatotoxic effect of an overdose of acetaminophen (AAP), an analgesic antipyretic drug; dimethylformamide (DMF), an industrial solvent; and sterigmatocystin (SMC), an anticancer drug is eliminated or significantly reduced by oral PTU treatment in the rat. Under the experimental conditions, liver necrosis was associated with hepatic GSH depletion. The basal GSH content was significantly higher in PTU-treated rats, but also in surgically thyroidectomized rats. Although both groups had increased resistance against AAP toxicity, PTU had a much stronger protective effect. Hepatic necrosis, the degree of GSH depletion and covalent binding of reactive metabolite(s) were related. Our data suggest multiple mechanism for the protective effect of PTU: (1) A relatively weak protective effect of induced hypothyroidism, (2) A stronger protective effect by a direct interaction of PTU on AAP metabolic pathways, (3) Increased tissue resistance. We hypothesize that the protective effect of PTU against a number of hepatotoxins is mediated by a common mechanism. The objectives of this proposal are to study the mechanism of the protective effect of PTU against AAP toxicity by in vitro and in vivo studies in the rat model. We plan to investigate: (1) the effect of PTU on GSH metabolism, (2) to separate the effects of the antithyroid properties of PTU from its direct effects on PTU toxicity with the emphasis on its GSH sparing properties, (3) to study the PTU effect on AAP intermediary metabolism, (4) its effect on drug kinetics, (5) its effect on tissue sensitivity to the toxic metabolite.